THE FUNCTIONS OF THE ESSENTIAL ELEMENTS 4-25 



relationships and interactions which exist between the different vital 

 activities, the cessation or abnormal performance of any one function 

 may influence all the other functions as well (Sect. 4), so that the vital 

 activity upon which the most marked external result is produced may be 

 one in which the element in question takes no direct part. Thus, to take 

 a concrete example, when growth is inhibited, the consumption and hence 

 also the translocation of carbohydrates ceases, so that if the assimilation 

 of carbon dioxide is possible the assimilatory products will accumulate in 

 the leaves until the inhibitory limit is reached, and this result will be pro- 

 duced whether the stoppage of growth is due to a deficiency of potassium 

 or phosphorus or to widely different causes. Similarly the absence of iron 

 probably inhibits the formation of chlorophyll only in an indirect manner, 

 for chlorophyll itself contains no iron. 



Owing to the intimate correlations which exist between the different 

 vital activities, an element essential only to a single function may exercise 

 various stimulatory influences upon others, but it is incorrect to suppose 

 that the ash constituents are of importance solely as stimuli to metabolic 

 activity \ for they undoubtedly form a constituent part of the protoplasm 

 and influence by their respective affinities the course of various metabolic 

 changes. The necessary supply of energy is for the most part obtained 

 by the oxidation of carbohydrates, for the ash constituents are, as a general 

 rule, absorbed in the form of compounds incapable of further oxidation. 

 A few organisms, however, obtain energy from the oxidation of inorganic 

 compounds, as is the case with the sulphur-bacteria and with nitrifying 

 organi ms (Sect. 63). 



No other element is capable of the variety of molecular combinations 

 possible to carbon, and hence it is easy to understand why, under terrestrial 

 conditions, carbon forms the most important and predominant element in all 

 the compounds which build up the living organism, and which by their 

 oxidation provide the necessary supply of energy. It is easy to under- 

 stand that only those elements can have attained universal importance 

 which have always been at the plant's disposal, but nevertheless though 

 sodium enjoys a world-wide distribution, and is always absorbed by plants, 

 it has not become an essential element. The same is the case with silicon, 

 the most widely distributed of all elements, which in spite of its comparative 

 insolubility may accumulate in large amount in certain plants. 



Why the necessities of living organisms have developed as described, 

 and why only certain of the universally present elements have become 

 essential, remains at present a mystery, for neither from the known 

 chemical or physical properties of these elements, such as their position in 



1 This frequently-repeated idea has been recently combated by Gustavson (Bot. Jahresb.. 1882, 

 P- 38). 



